Fix Loading on pages (microsoft#290)

This subtly regressed in microsoft#244, so subtly that I didn't really notice it till I fixed it.

In that PR, we replaced the `visibility` binding of the `ProgressBar` with a binding to `IsInitialized`. For most pages, this actually just works fine - they're only initialized once they return some results. And most pages are only loading until they first return something.  

But the trick is with pages that do some `Loading` _after_ they're initialized. For them, the progress bar never shows up, because `Initialized` is already true.

I actually only stumbled upon this because of a totally different bug (which this also fixes). 

In rare cases we'd load a little out of order, and the ShellPage would try to determine if it should hide the `ErrorMessage` or not. About 25% of the time, it would evaluate `{x:Bind ViewModel.CurrentPage.ErrorMessage, Converter={StaticResource StringNotEmptyToVisibilityConverter}, Mode=OneWay}` _before_ the `CurrentPage` got set, so it would show it initially. That looked real janky.

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/LoadingPageViewModel.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CmdPal.Extensions;
+
+namespace Microsoft.CmdPal.UI.ViewModels;
+
+public partial class LoadingPageViewModel : PageViewModel
+{
+    public LoadingPageViewModel(IPage? model, TaskScheduler scheduler)
+        : base(model, scheduler)
+    {
+        ModelIsLoading = true;
+        IsInitialized = false;
+    }
+}

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/PageViewModel.cs

@@ -15,7 +15,10 @@ public partial class PageViewModel : ExtensionObjectViewModel, IPageContext
 
     private readonly ExtensionObject<IPage> _pageModel;
 
+    public bool IsLoading => ModelIsLoading || (!IsInitialized);
+
     [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(IsLoading))]
     public partial bool IsInitialized { get; protected set; }
 
     [ObservableProperty]
@@ -38,7 +41,9 @@ public partial class PageViewModel : ExtensionObjectViewModel, IPageContext
 
     public string Title { get => string.IsNullOrEmpty(field) ? Name : field; protected set; } = string.Empty;
 
-    public bool IsLoading { get; protected set; } = true;
+    // This property maps to `IPage.IsLoading`, but we want to expose our own
+    // `IsLoading` property as a combo of this value and `IsInitialized`
+    public bool ModelIsLoading { get; protected set; } = true;
 
     public IconDataType Icon { get; protected set; } = new(string.Empty);
 
@@ -91,13 +96,14 @@ public override void InitializeProperties()
         }
 
         Name = page.Name;
-        IsLoading = page.IsLoading;
+        ModelIsLoading = page.IsLoading;
         Title = page.Title;
         Icon = page.Icon;
 
         // Let the UI know about our initial properties too.
         UpdateProperty(nameof(Name));
         UpdateProperty(nameof(Title));
+        UpdateProperty(nameof(ModelIsLoading));
         UpdateProperty(nameof(IsLoading));
         UpdateProperty(nameof(Icon));
 
@@ -143,7 +149,8 @@ protected virtual void FetchProperty(string propertyName)
                 this.Title = model.Title ?? string.Empty;
                 break;
             case nameof(IsLoading):
-                this.IsLoading = model.IsLoading;
+                this.ModelIsLoading = model.IsLoading;
+                UpdateProperty(nameof(ModelIsLoading));
                 break;
             case nameof(Icon):
                 this.Icon = model.Icon;

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/SettingsViewModel.cs

@@ -33,7 +33,7 @@ public SettingsViewModel(SettingsModel settings, IServiceProvider serviceProvide
 
         Icon = new("\uE713");
         IsInitialized = true;
-        IsLoading = false;
+        ModelIsLoading = false;
         Title = "Settings";
 
         var activeProviders = GetCommandProviders();

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ShellViewModel.cs

@@ -24,7 +24,7 @@ public partial class ShellViewModel(IServiceProvider _serviceProvider, TaskSched
     public partial bool IsDetailsVisible { get; set; }
 
     [ObservableProperty]
-    public partial PageViewModel? CurrentPage { get; set; }
+    public partial PageViewModel CurrentPage { get; set; } = new LoadingPageViewModel(null, _scheduler);
 
     [RelayCommand]
     public async Task<bool> LoadAsync()
@@ -98,7 +98,7 @@ public void LoadPageViewModel(PageViewModel viewModel)
                         {
                             var result = (bool)viewModel.InitializeCommand.ExecutionTask.GetResultOrDefault()!;
 
-                            CurrentPage = result ? viewModel : null;
+                            CurrentPage = viewModel; // result ? viewModel : null;
                             ////LoadedState = result ? ViewModelLoadedState.Loaded : ViewModelLoadedState.Error;
                         },
                         CancellationToken.None,

src/modules/cmdpal/Microsoft.CmdPal.UI/ShellPage.xaml

@@ -131,7 +131,7 @@
                 Grid.ColumnSpan="2"
                 VerticalAlignment="Bottom"
                 IsIndeterminate="True"
-                Visibility="{x:Bind ViewModel.CurrentPage.IsInitialized, Converter={StaticResource BoolToInvertedVisibilityConverter}, Mode=OneWay}" />
+                Visibility="{x:Bind ViewModel.CurrentPage.IsLoading, Mode=OneWay}" />
 
             <Grid
                 x:Name="ContentGrid"